The present invention relates to a connecting method of different kind optical fibers for connecting the different kind optical fibers having different mode field diameters (MFDs) to each other.
In recent years, an increase in the capacity of an optical communication system is required. A dispersion management line for performing high bit-rate communication in a band of e.g., 1550 nm in wavelength is vigorously considered to satisfy this requirement. For example, this dispersion management line is formed by connecting a single mode optical fiber such as a 1300 nm zero dispersion optical fiber, and a dispersion-compensating optical fiber for compensating dispersion and a dispersion slope of this single mode optical fiber.
For example, the above dispersion-compensating optical fiber is formed by a DCF (Dispersion Compensating Fiber), a DSCF (Dispersion Slope Compensating Fiber), an RDF (Reverse Dispersion Fiber), etc. When such a dispersion management line is used in an optical submarine cable, etc., it is desirable to connect different kind optical fibers with low transmission loss and high strength.
For example, the 1300 nm zero dispersion optical fiber is a single mode optical fiber, and the MFD of the 1300 nm zero dispersion optical fiber at a wavelength of 1550 nm ranges from 9 to 11 xcexcm. Further, the MFD of an MFD enlarged type single mode optical fiber at the wavelength of 1550 nm is 11 xcexcm or more.
In contrast to this, the MFD of the dispersion-compensating optical fiber having negative high dispersion characteristics at the wavelength of 1550 nm is about 5 xcexcm. Accordingly, the MFD of the dispersion-compensating optical fiber is small in comparison with the single mode optical fiber. The dispersion-compensating optical fiber has a high relative refractive index difference A of about 3% and a small core diameter of 2 to 3 xcexcm.
When the above different kind optical fibers of different MFDs are connected to each other, splice loss is increased by the difference in MFD. For example, the splice loss is about 1.94 dB when the single mode optical fiber having 10 xcexcm in MFD and the dispersion-compensating optical fiber having 5 xcexcm in MFD are connected to each other by conforming their optical axes.
For example, processing using a TEC method (Thermally Diffused Expanded Core) is performed to restrain this increase in splice loss after connecting end faces of the different kind optical fibers are connected to each other. For example, the processing using the TEC method is performed by heating connecting portions of the single mode optical fiber and the dispersion-compensating optical fiber. The processing using the TEC method is processing for enlarging the MFD of the dispersion-compensating optical fiber by diffusing Ge (dopant) within a core and conforming this MFD to the MFD of the single mode optical fiber by heating the above connecting portions. The splice loss is greatly reduced by performing this processing.
FIG. 9 is a flow chart showing one example of a conventional connecting process of the different kind optical fibers. For example, the different kind optical fibers such as the single mode optical fiber and the dispersion-compensating optical fiber are connected to each other as follows in accordance with the flow shown in FIG. 9. A step 101 of FIG. 9 is a coating removing process for removing the coating layers of respective connecting end portions of the different kind optical fibers and exposing the connecting end portions of the optical fibers. A step 102 is a fiber cleaning process for cleaning an exposed portion of each of these optical fibers. A step 103 is a process for forming a protecting layer on the surface of the exposed portion of each cleaned optical fiber.
A step 104 is a process for cutting a tip portion of each optical fiber. A step 105 is a process for fusion-splicing the cut different kind optical fibers by butting their connecting end faces. The above cutting processing of the optical fiber and the fusion splice processing of the optical fiber are performed by arranging the exposed portion (tip portion) of the optical fiber in e.g., a V-groove formed in a fixing member.
The exposed portion of the optical fiber arranged in the V-groove is positioned and fixed to the fixing member by pressing this optical fiber from its upper side by a clamp member. In such a gripping fixing state (glass gripping state), the cutting processing and the fusion splice processing of the above optical fiber are performed. The protecting layer formed on the surface of the exposed portion of the optical fiber in the above step 103 is arranged to reduce a bad influence on the optical fiber due to the gripping fixation.
A step 106 is a process for taking heat treatment for conforming the MFDs of the connecting end portions of the optical fibers connected to each other.
A small crack is caused in a surface portion of the connecting portion of the different kind optical fiber by heating of the fusion splice processing and the heat treatment. When there is such a crack, strength of the connecting portion of the different kind optical fiber is reduced. Therefore, an operation for preventing this strength deterioration is performed. This operation is performed by the etching process of a step 107, and is an operation for removing the crack by etching the surface portion of the connecting portion of the optical fiber after the above heat treatment.
A step 108 is a recoating process for coating the connecting portion of the different kind optical fiber by forming a coating layer on the surface of the exposed portion of the optical fiber. A step 109 is a process for inspecting whether or not the different kind optical fiber is connected in accordance with a standard by a proof test.
However, in a conventional method, the connecting portions of the single mode optical fiber and the dispersion-compensating optical fiber were about 1 GPa in average breaking strength, and no satisfactory strength was obtained. Further, this strength was greatly dispersed.
Therefore, the present inventors examined the cause of the strength deterioration of the connecting portion of the different kind optical fiber. As a result, it has been found that one of causes of the strength deterioration of the connecting portion of the different kind optical fiber resides in an environment in the heat treatment for conforming the mode field diameters of the respective optical fibers in the connecting portions.
Namely, when the heat treatment is taken, foreign matters floated around the connecting portion of the different kind optical fiber and foreign matters attached to the surface of the connecting portion are burned onto the surface of the connecting portion of the different kind optical fiber by the heat treatment. Therefore, there is a case in which a small crack is caused in the connecting portion of the different kind optical fiber by these burned foreign matters. Thus, it has been found that the strength of the connecting portion of the different kind optical fiber is deteriorated by this crack.
In the conventional method, processing for removing the crack from a surface portion of the connecting portion of the different kind optical fiber by etching this surface portion is performed after the heat treatment. However, it is confirmed that no crack caused by the burned foreign matters due to the heat treatment can be almost removed by this etching processing.
Further, the present inventors have also found the following matters by examining the cause of the strength deterioration of the connecting portion of the different kind optical fiber. The heat treatment of the connecting portion of the different kind optical fiber was conventionally taken after the protecting layer was formed on the surface of an exposed portion of the optical fiber. Therefore, the protecting layer formed in the connecting portion of the different kind optical fiber was burned and a large amount of foreign matters was generated at a heat treatment time of the connecting portion of the different kind optical fiber. These foreign matters were burned onto the optical fiber surface and many small cracks were caused.
As mentioned above, no cracks caused by the burned foreign matters due to the heat treatment can be almost removed by the etching processing after the heat treatment. Therefore, the strength of the connecting portion of the different kind optical fiber is deteriorated by these unremoved cracks.
The present invention in its one aspect provides a connecting method of different kind optical fibers able to obtain a connecting state of the different kind optical fibers with low splice loss and satisfactory strength.
The present invention provides a connecting method of different kind optical fibers having the following construction. Namely, the connecting portions of the different kind optical fibers of the present invention is a connecting method of different kind optical fibers for performing plural processings including processing for mutually fusion-splicing connecting end faces of the different kind optical fibers of different mode field diameters, and heat treatment for heating their connecting portions and conforming the mode field diameters of the respective optical fibers in these connecting portions; wherein at least said heat treatment among the plural processings in a connecting process of the different kind optical fibers is taken within a low dust space having a clean degree of 1000 or less in class.
In a preferable mode example of the present invention, processing for cleaning at least a portion of a heat treatment object is performed just before the heat treatment is taken.
In a more preferable mode example of the present invention, a protecting layer is formed on the surface of each of the optical fibers after the heat treatment by avoiding a heat-treated area, and an optical fiber surface portion of an etching processing area including said heat-treated area is then etched.
Further, in a more preferable mode example of the present invention, the optical fibers connected to each other are respectively coated with coating layers, and fusion splice is performed after the coating layers of connecting end portions of these respective optical fibers are removed, and the length interval of the etching processing area is a length interval equal to or greater than a length interval for taking the heat treatment and smaller than an interval for removing the coating layers therefrom.
Further, another mode example of the present invention resides in a connecting method of different kind optical fibers for connecting the different kind optical fibers of different mode field diameters having fiber outer circumferences coated with coating layers, wherein the optical fibers are exposed by removing the coating layers of connecting end portions of the optical fibers, and connecting end faces of the different kind optical fibers are then mutually butted and fusion-spliced, and heat treatment for heating these connecting portions and conforming the mode field diameters of the respective optical fibers in these connecting portions is then taken, and at least a root outer circumferential face of the exposed optical fiber is then coated with a protecting layer together with a removing end face portion of the coating layer, and a heat-treated optical fiber surface portion is then etched.
Further, in one mode example of the present invention in another aspect of the present invention, processing for cleaning at least a portion of a heat treatment object is performed just before the heat treatment is taken.
Further, in one mode example of the present invention in another aspect of the present invention, the above etching processing interval includes a heat treatment interval, and a non-etching processing interval is formed between the etching processing interval and coating layer removing end faces on both sides of this etching processing interval.